A clear sunny day occurs when a warm, dry air mass moves in with high pressure, low humidity, and lots of sunshine. Light rain happens when a warm air mass meets a cold air mass with moderate humidity and some wind. Severe rainstorms require a collision between two air masses with large temperature differences that produces heavy wind and high humidity. Light snow falls from a cold air mass with low temperatures and moderate humidity, while heavy snow needs very cold air, high humidity, and strong winds from clashing air masses.